"Arfa Daily" wrote in message
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"abrsvc" wrote in message
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On May 10, 9:19 pm, "Arfa Daily" wrote:
"Gary Woodruff" wrote in message

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Hello all, I have a Teac AG500 that will not switch between inputs.
The
CD and Phono indicator lights are both lit. When you try to select
tuner
the indicator lights momentarily then goes out(leaving CD and Phono
lit). It does play audio through the CD input. I have checked all
resistors and tact switches on the front panel board all are good. I
am
leaning towards the IC on the mainboard that switches the inputs but
do
not have a print and am not sure which IC it is! Any and all help is
greatly appreciated.

Thanks, Gary

It's a nice thought, but I would be leaning more towards it being the
system
control micro, as there is an issue with the function indicator LEDs, as
well as the actual switching. It will be the micro that is responsible
for
generating both the LED drive signals, which will likely be linear
one-for-one, and the control signal for the source switching IC, and
this
signal will probably be I2C.

Have you looked with a 'scope at the actual matrix lines for the front
panel
tacts ? As well as stuck switches, I have also had leaky switches, and
leaky
caps across switches. If it is one of the systems that uses a resistor
ladder and an A-D input on the micro to determine which button has been
pressed, these can get mightily upset with leakage amongst the wanted
resistance. I know you say that you have checked the Rs on the front
panel,
but have you checked every one carefully for value ? Do other switches
do as
they should ?

Arfa

Arfa,

While I agree that your diagnosis is possible, experience tells me
that the switching IC is the most likely culprit here. I would
mention that checking for filter caps on the 5V line would also be a
good idea.

Dan


With the majority of input switching schemes that I see on hifi equipment,
the front end selector IC is responsible for for just signal switching,
and is an I2C driven device. Indicator LEDs are almost exclusively driven
in a fully linear fashion, straight from a port on the system control
micro or, if there is enough of them, via a bus expander IC. I'm
struggling to see how a defective input switching IC, would affect the
function indicator LEDs in the way the OP described, unless you're
thinking that the LEDs on that model - which I admit that I don't have
schematics for - might be being driven by a few 'spare' channels on the
switching IC ? Looking at, for instance, the AR600, LED function indicator
switching is completely separately handled to signal switching. The LEDs
are driven directly by the micro. Of course, if the AG 500 is well known
to you, then I bow to your knowledge of it, and its common faults. I agree
with you that checking the 5 V rail for level and noise, would also be a
good move.

Arfa

Some input switch IC's do also run front panel LED's. Saw one just the other
day. Can't remember the IC number, though.

Mark Z.